The present invention relates to a non-woven fabric for artificial leather and to artificial leather produced from the same and, more specifically, to a non-woven fabric formed of fine fibers obtained from a strippable and splittable composite short fiber comprising at least two components and to artificial leather produced from the same.
In recent years, artificial leather which is a natural leather substitute has been widely used in the fields of garments, general materials and sports because its characteristic features such as lightweight and easy care have been recognized by consumers. However, artificial leather having improved softness which is the characteristic feature of natural leather and drapeability derived from a fine structure has been demanded from the market and various proposals have been made.
For example, there is proposed a process in which the fineness of a fiber forming a non-woven fabric is reduced to 0.3 denier or less. Artificial leather produced from this fiber is actually produced and marketed. When a non-woven fabric formed of fibers of 0.3 denier or less (to be referred to as xe2x80x9cnon-woven microfabricxe2x80x9d hereinafter) is obtained simply by reducing the monofilament size of the fibers, neps or the like are formed in the carding step with the result of a reduction in process efficiency. Therefore, various processes which improve this are proposed. These conventional production processes are roughly divided into the following three groups.
As disclosed by JP-B 48-22126 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d), the first group uses a sea-island type composite short fiber having such a cross section that a sea and many islands are formed from a sea component and an island component incompatible with the sea component by the shapes of spinning nozzles, respectively. In this process, a non-woven fabric is produced by carrying out a mechanical entangling treatment such as needle punching or contact with a jet liquid flow after the conventional production process of a non-woven fabric. Thereafter, the non-woven fabric is impregnated with an elastic polymer, or a non-woven microfabric is formed by dissolving and removing the sea component with a solvent which dissolves the sea component but not the island component before impregnation, and an artificial leather substrate is produced using this non-woven fabric as a base.
As disclosed by JP-B 48-27443, the second group uses a polymers-blended sea-island type composite short fiber obtained by mixing a sea component for forming the sea and an island component for forming islands incompatible with the sea component in the cross section of the fiber in a molten state and spinning a dispersion containing the island component dispersed in the sea component. Also in this process, like the above sea-island type composite short fiber, after a non-woven fabric is formed, the sea component is dissolved and removed with a solvent which dissolves the sea component but not the island component to produce a non-woven microfabric, and an artificial leather substrate is produced using this non-woven fabric as a base.
As disclosed by JP-A 4-65567 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), the third group uses a strippable and splittable composite short fiber having such a cross section that two different components incompatible with each other are arranged alternately several times (as side-by-side type). In this process, the strippable and splittable composite short fiber is stripped and split into fine fibers while they are mechanically entangled by contact with a jet liquid flow or the like to produce a non-woven microfabric. Thereafter, the non-woven microfabric is impregnated with an elastic polymer to produce an artificial leather substrate comprising the non-woven microfabric as a base.
As disclosed by JP-A 49-26581, 49-93663, 49-132377 and 54-96181, there is still another process in which heat shrinkability is provided to a polyester-based resin component to facilitate the stripping and splitting of a strippable and splittable composite short fiber comprising a polyamide component and a polyester-based resin component.
Suede type and nubuck type artificial leathers produced from non-woven microfabrics formed of these fibers are very soft and have a good appearance making use of the small monofilament sizes of the fibers. However, when a grain type artificial leather is produced by forming a film of an elastic polymer on the surface, it is not satisfactory because it is not so tight as natural leather and is greatly wrinkled when its surface is bent inward. The reason for this is that even when single fiber having a small fineness is formed by splitting parent fiber, it is entangled in the state of a large assembly because the fineness of the parent fiber for forming the single fiber having a small fineness is 3 to 10 denier with the result of the formation of spaces of the same size as spaces formed in a non-woven fabric formed by entangling single fiber having a large fineness of the prior art.
As described above, since the beauty of artificial leather, mainly suede type artificial leather comprising a non-woven microfabric as a base, has gained wide acceptance from consumers, great progress is being made. A grain type artificial leather has softness even when a non-woven microfabric is used and a grain layer is formed on the surface, but it lacks tightness and is easily wrinkled by bending. When it is formed into a shoe, trunk, glove or furniture, or it is used or worn, it is difficult to obtain an aesthetic appearance and the improvement of the appearance has been strongly desired from the market.
The present inventors paid attention to the fact that the cause of bending wrinkles is the structure of a non-woven fabric formed by entangling the above assembly of fibers having a small fineness and began to study how a fiber entangled state is finely and uniformly formed in the structure of a non-woven fabric formed by entangling fibers having a small fineness and the characteristic properties of the non-woven fabric when formed. A first possible means is to use a short fiber having a small fineness. However, with this means, neps are formed in the carding step because the fiber is very fine, thereby reducing process efficiency. Therefore, the means was excluded from the list of study.
After a process for producing a non-woven microfabric from a composite short fiber from which fibers having a small fineness can be formed was studied, in consideration of the facts that a process for dissolving and removing a sea component from a sea-island type composite short fiber and a polymers-blended sea-island type composite short fiber is required and that there is the loss of a raw material which is dissolved and removed, a non-woven microfabric having an entangled structure of fibers having a small fineness formed from a strippable and splittable composite short fiber which is economically advantageous has been studied. A non-woven fabric obtained from a conventional strippable and splittable composite short fiber by a jet liquid flow contact entangling method cannot have a uniform and fine structure but a structure that stripped and split fibers having a small fineness are mostly entangled in the state of a large assembly. In the above process which uses a strippable and splittable composite short fiber comprising a heat shrinkable polyester component, the shrinkage energy of the polyester component is consumed at the time of stripping and splitting and an assembly of fibers having a small fineness is not broken because the process is aimed to facilitate stripping making use of the axial shrinkage force of the polyester component at the time of stripping and splitting. As a result, a uniform and fine structure cannot be obtained.
It is therefore a first object of the present invention to provide a non-woven fabric having such a structure that fibers having a small fineness formed from a strippable and splittable composite short fiber are entangled with one another as uniformly and finely as possible by making the proportion of fiber assemblies as small as possible, and a production process for the same.
It is a second object of the present invention to provide a non-woven fabric having such a structure that fibers having a small fineness formed from a strippable and splittable composite short fiber are finely and uniformly entangled with one another and hence, spaces between fibers are small on the average and the distribution of the spaces is relatively small, and a production process for the same.
It is a third object of the present invention to provide a sheet for artificial leather which is very soft and tight and has a fine structure that it is rarely wrinkled by bending, and a production process for the same.
It is a fourth object of the present invention to provide a process for producing the above non-woven fabric and sheet industrially advantageously.
It has been found from studies conducted by the present inventors that the above objects of the present invention are attained by the following non-woven fabric.
That is, according to the present invention, there is provided a non-woven fabric formed of fine fibers which satisfies the following requirements:
(i) the fine fibers should be obtained by splitting a strippable and splittable composite short fiber comprising at least two resin components which are incompatible with each other;
(ii) the fine fibers should have a monofilament size of 0.01 to 0.5 denier;
(iii) the fine fibers should form a fine non-woven fabric structure that they are entangled with one another at random;
(iv) the apparent density should be 0.18 to 0.45 g/cm3;
(v) the average area of spaces between fibers in the cross section of the non-woven fabric measured by the image analysis of an electron scanning microscope should be 70 to 250 xcexcm2; and
(vi) the non-woven fabric should have such a uniform structure that the standard deviation of the area of a space between fibers in the cross section of the non-woven fabric measured by the image analysis of the electron scanning microscope is 200 to 600 xcexcm2.
It has also been found from studies conducted by the present inventors that the non-woven fabric of the present invention can be obtained by the following production process.
That is, according to the present invention, there is provided a process for producing a non-woven fabric which comprises the steps of:
(1) forming card webs from a strippable and splittable composite short fiber comprising at least two resin components which are incompatible with each other and at least one of which is heat shrinkable and layering together the card webs (layering step);
(2) entangling and stripping/splitting the obtained layered web to split the composite short fiber into fine fibers having a monofilament size of 0.01 to 0.5 denier and entangle the fine fibers with one another so as to produce an unshrunk non-woven fabric (entangling and splitting step); and
(3) heating the obtained unshrunk non-woven fabric to thermally shrink heat shrinkable fine fibers contained in the fine fibers to reduce the area of the non-woven fabric by 10 to 50% (shrinking step).
The present invention will be described in detail hereinunder.
At least two components forming the strippable and splittable composite short fiber of the present invention have fiber formability and may be any combination of synthetic resins if they are not compatible with each other. However, in consideration of process control and productivity in the production of a strippable and splittable composite short fiber, polyester-based resins and polyamide-based resins which allow for melt spinning can be advantageously used.
That is, the synthetic resins used to produce the strippable and splittable composite short fiber of the present invention are not particularly limited if they are two incompatible components selected from fiber forming polyester-based resins and fiber forming polyamide-based resins. The polyester-based resins include polyethylene terephthalate, polybutylene terephthalate and the like, and the polyamide-based resins include nylon-6, nylon-66, nylon-12 and the like. Out of these, a combination of polyethylene terephthalate and nylon-6 is preferred from the viewpoint of process efficiency and cost.
The strippable and splittable composite short fiber may comprise three components including a polyester copolymer resin containing a metal salt sulfonate as another polyester-based resin component.
The strippable and splittable composite short fiber of the present invention has such a structure that at least one of the constituent components is split into two or more parts and at least part of each constituent component is exposed to the surface of the fiber in the cross section of the fiber. The number of split parts is not-particularly limited but preferably 8 to 24 in consideration of process efficiency and strippability/splittability. The proportion of one component of the strippable and splittable composite short fiber of the present invention is preferably 30 to 70 wt %, particularly preferably 40 to 60 wt % based on the total from the viewpoint of the splittability and spinnability of the fiber. Above this range, it is difficult to control the balance of the viscosity of the resin, which might cause a section failure and reduce the splitting rate.
The strippable and splittable composite short fiber of the present invention is preferably a composite fiber comprising a polyester component and a polyamide component, wherein heat shrinkage ratio of said polyester component is 10% or more larger than that of the polyamide component. The present invention is characterized in that a non-woven fabric in which fine fibers obtained after stripping and splitting are entangled with one another into a fiber assembly in the prior art is made uniform and fine by thermally shrinking after stripping and splitting to provide freedom between a polyester fiber and a polyamide fiber having small shrinkage through the shrinkage of the polyester fiber arranged alternately to alleviate the assembly of fibers and by thermally shrinking the whole non-woven fabric. Therefore, the difference of heat shrinkage ratio between the polyester component and the polyamide component must be 10% or more. When the difference is smaller than 10%, the effect of the present invention cannot be obtained.
The above difference of heat shrinkage ratio between the components of the strippable and splittable composite short fiber of the present invention can be obtained by controlling the spinning temperature, take-up rate, drawing temperature and draw ratio. The spinning temperature is suitably determined in consideration of balance between the viscosities of the both components. There is a tendency that when spinning is carried out at low temperatures, fibers having a large difference of heat shrinkage ratio are obtained. The take-up rate of filaments is preferably 2,000 m/min or less. When the take-up rate is higher than 2,000 m/min, the crystalization by orientating a fiber proceeds and a sufficiently large difference of heat shrinkage ratio may not be obtained.
The stripped and split fibers of the present invention have a fineness of 0.01 to 0.5 denier. When the fineness is smaller than 0.01 denier, the fibers adhere to each other after stripping and splitting because the fibers are too fine, thereby making it difficult to impregnate an elastic polymer, which is not preferred for the production of artificial leather. When the fineness is larger than 0.5 denier, a non-woven fabric having a uniform and fine structure which the present invention is directed to cannot be obtained because the fibers are too thick. The fineness of a filament (parent fiber) forming the fibers having the above fineness which is determined by the number of split parts, fineness after stripping and splitting and draw ratio is preferably 1 to 10 denier. When the fineness of the filament is smaller than 1 denier, end breakage readily occurs at the time of spinning, resulting in a reduction in productivity. When the fineness is larger than 10 denier, the fineness of a product becomes large and the obtained non-woven fabric hardly has a uniform and fine structure which the present invention is directed to even when the strippable and splittable composite short fiber is split.
There is a tendency that fibers having a larger difference of heat shrinkage ratio are obtained as the drawing temperature becomes lower. The difference of heat shrinkage ratio becomes larger as the draw ratio decreases. When the drawing temperature and the draw ratio are increased, the crystalization by orientating the fibers is promoted and the targeted difference of heat shrinkage ratio is not obtained. Especially in the present invention, the drawing temperature is preferably 40 to 60xc2x0 C. and the draw ratio is preferably 1.0 to 3.0 times. When the drawing temperature is lower than 40xc2x0 C., the fiber strength becomes weak and the card passability deteriorates, and when the drawing temperature is higher than 60xc2x0 C., a sufficiently large difference of heat shrinkage ratio is hardly obtained. When the draw ratio is smaller than 1.0 time, satisfactory fiber characteristic properties are not obtained and when the draw ratio is larger than 3.0 times, a sufficiently large difference of heat shrinkage ratio is hardly obtained. The draw ratio is more preferably 1.2 to 2.5 times.
A lubricant or the like is applied to the surface of the strippable and splittable composite fiber thus obtained, and the fiber is crimped, dried and cut to a predetermined length by a cutter or, the like. Drying is generally carried out with hot air or the like. As the drying temperature becomes lower, fibers having a larger difference of heat shrinkage ratio are apt to be obtained. The drying temperature is preferably 70xc2x0 C. or less, more preferably 40 to 60xc2x0 C. When the drying temperature is higher than 70xc2x0 C., the targeted difference of heat shrinkage ratio is not obtained and when the drying temperature is lower than 40xc2x0 C., the drying efficiency is low, which is not practical from the viewpoint of productivity and cost. The length of each fiber is preferably 30 to 100 mm, more preferably 40 to 70 mm in consideration of card passability. When the length of the fiber is larger than 100 mm, the card passability of the fiber deteriorates and when the length is smaller than 30 mm, it becomes difficult to card the fiber.
The strippable and splittable composite short fiber thus obtained is opened with an ordinary roller card to form a web. At this point, other short fiber may be blended. However, to attain the object of the present invention, the proportion of the other short fiber to be blended is preferably less than 40 wt %. More preferably, a short fiber substantially formed from the strippable and splittable composite short fiber of the present invention is formed into a web. When the proportion of the other short fiber to be blended is 40 wt % or more, it may be difficult to obtain a non-woven fabric having a uniform and fine structure which the present invention is directed to.
The other fiber to be blended is not particularly limited but at least one may be selected from regenerated fibers of rayon and the like, semi-synthetic fibers of acetate and the like, natural fibers of wool and the like, polyamide fibers such as nylon-6 and nylon-66 fibers, polyester-based fibers such as polyethylene terephthalate and polybutylene terephthalate fibers, and polyolefin-based fibers such as polyethylene and polypropylene fibers and used. As a matter of course, the shape of each fiber is not limited and a core-sheath composite fiber formed from a combination of the above thermoplastic resins, strippable and splittable composite short fiber, short fiber having a modified cross section and the like may be used.
The card webs obtained as described above are layered together to a target weight with a cross layer or the like to produce a layered web which is then subjected to a mechanical entangling treatment. The entangling treatment of the layered web is carried out by a conventionally known method per se for entangling fibers by punching with barbed needles or by contact with a jet liquid flow. Since the strippable and splittable composite short fiber must be entangled three-dimensionally and treated so that it can be stripped and split, it is the most effective to carry out a jet liquid flow contact entangling treatment after needle punching. For example, to obtain a non-woven fabric having a weight of 150 g/m2, sharped water may be sprayed onto the front and rear sides of a non-woven fabric at a water pressure of 50 to 200 kg/cm2 from a nozzle having orifices with a hole diameter of 0.05 to 0.5 mm at intervals of 0.5 to 1.5 mm one time to four times each. When this jet liquid flow contact treatment is carried out, the non-woven fabric may be dried at a temperature that its shrinkage performance remains in hot water heated at 50xc2x0 C. or more.
The unshrunk non-woven fabric thus entangled and stripped/split is shrunk by heating. By heating the non-woven fabric obtained by entangling an assembly of stripped and split fibers having a small fineness, the form of the assembly is broken and randomized because a polyester fiber forming the assembly has a larger shrinkage ratio than that of a polyamide fiber and shrinkage occurs in the plane direction, thereby increasing density. Thus, by heating a conventional non-woven microfabric formed by entangling an assembly of fibers having a small fineness, one component forming the assembly and arranged alternately thermally shrinks and breaks the structure of the assembly with the result of the formation of a fine structure that the fibers having a small fineness are entangled with one another at random, thereby making uniform the whole structure and increasing density. As a result, compared with the conventional non-woven microfabric, the volume of a space between fibers formed by entangling fibers is fined. That is, the volume of the space formed between the fibers becomes smaller and the number of the spaces becomes larger than those of the conventional non-woven microfabric, thereby making the whole structure uniform and fine.
Heating for shrinking the unshrunk non-woven fabric may be either wet heating or dry heating but the unshrunk non-woven fabric is preferably shrunk in hot water. When the unshrunk non-woven fabric is shrunk in hot water, it is shrunk while its tension is alleviated by its buoyancy, thereby making it easy to form a non-woven fabric structure of interest effectively. Therefore, the temperature of the hot water is preferably 65 to 90xc2x0 C., more preferably 67 to 72xc2x0 C. When the heating temperature is lower than 65xc2x0 C., heat shrinkage becomes insufficient and when the heating temperature is higher than 80xc2x0 C., the shrinking speed becomes fast, thereby making it difficult to realize uniform heat shrinkage.
The area of the non-woven fabric is shrunk by the heat shrinkage of the polyester fiber, thereby increasing the density. When the area shrinkage ratio at this point is obtained from {(area before shrinkagexe2x88x92area after shrinkage)/(area before shrinkage)}xc3x97100 (%), it is preferably 10 to 50%, more preferably 15 to 40%. When the area shrinkage ratio is smaller than 10%, the non-woven fabric having a fine and uniform structure of the present invention cannot be obtained. When the area shrinkage ratio is larger than 50%, the non-woven fabric is wrinkled at the time of heat shrinkage and the space between fibers becomes too small, that is, the apparent density becomes higher than required. As a result, a non-woven fabric which is tight but inferior in drapeability is obtained disadvantageously.
As the area shrinkage ratio becomes larger, a non-woven fabric having a higher apparent density is obtained. The apparent density of the non-woven fabric of the present invention is preferably 0.18 to 0.45 g/cm3, more preferably 0.25 to 0.40 g/cm3. To develop the uniform non-woven fabric structure of the present invention by heat shrinkage, the lower limit of apparent density is 0.18 g/cm3. A non-woven fabric having an apparent density of more than 0.45 g/cm3 is tight but inferior in drapeability as described above.
The area shrinkage and the apparent density can be easily controlled with the heat shrinkage ratio, blend ratio and entangling degree of the polyester component of the strippable and splittable composite short fiber of the present invention or the heating temperature of the shrinking step.
The non-woven fabric of the present invention obtained as described above has such a structure that fibers are uniformly and finely entangled with one another. The average area of spaces between fibers in the cross section in a direction perpendicular to the surface of the non-woven fabric measured by the image analysis of an electron scanning microscope is 70 to 250 xcexcm2, preferably 100 to 230 xcexcm2. The standard deviation at this point is 200 to 600 xcexcm2, preferably 250 to 500 xcexcm . When the average area is smaller than 70 xcexcm a non-woven fabric having high density and a fine and uniform structure which cannot be obtained in the prior art is obtained but the non-woven fabric is tight but inferior in drapeability as described above. When the average area is larger than 250 xcexcm2 and a grain layer is formed on the surface of the non-woven fabric to produce artificial leather, the obtained leather is not tight and is easily wrinkled by bending like the leather by use of the prior art non-woven fabric though it looks uniform at first sight.
The value of standard deviation indicating uniformity is preferably smaller. When it is larger than 600 xcexcm2, large spaces are scattered even if the average value falls within the range of the present invention and a non-woven fabric which is easily wrinkled by bending is obtained disadvantageously.
The average area of spaces between fibers in the cross section in a direction perpendicular to the surface of the non-woven fabric of the present invention is measured by the following method of analyzing an image obtained by an electron scanning microscope.
(1) Formation of Sample;
A gold film is formed on a sectional sample of a non-woven fabric to be measured to a thickness of 800 xc3x85 by ion sputtering at a pressure of up to 10xe2x88x921 Pa using the JFC-1500 ion sputtering device of JEOL Ltd.
(2) Photographing with Electron Microscope;
The waveform of an image signal for the sample formed in (1) above is displayed on a CRT for observation at an acceleration voltage of 5 kV, a filament current of 2.2 A and a scanning speed of 15.7 sec/line (horizontal, 60 Hz) using the JSM-6100 electron scanning microscope of JEOL Ltd. to determine exposure by aligning the peak and the lowest level of the waveform with 5 V and 0 V of a potential scale and turning off a waveform monitor. The magnification is then set at 200X.
(3) Image Processing;
An image is input (automatically) from an electron scanning microscope using the IP-1000PC high-definition image analytical system of Asahi Chemical Industry Co., Ltd. and the image processing of xe2x80x9caperture measurementxe2x80x9d is selected for measurement. The binary threshold value of this image processing is xc2xd of the maximum value of a brightness distribution. The average area of spaces between fibers in the cross sections of the non-woven fabric and substrate for artificial leather of the present invention is measured by the method described above.
In the above measurements (1) to (3), other devices having the same functions and performance as the ion sputtering device, the electron scanning microscope and the image analyzer may be used.
The obtained non-woven fabric itself is suitably used for artificial leather as well as for other applications such as garments, interior finishes, interior materials, wipers such as industrial wipers and wiping cloth, and filters such as bag filters and filtration cloth.
The above non-woven fabric of the present invention is impregnated with an elastic polymer to produce a sheet which is very soft and tight and has great value as a base fabric for artificial leather.
According to studies conducted by the present inventors, there is provided the following sheet which is produced from the above non-woven fabric and useful as a base fabric for artificial leather. That is, according to the present invention, there is provided a sheet obtained by impregnating a non-woven fabric formed of fine fibers with an elastic polymer, which satisfies the following requirements:
(i) the fine fibers should be obtained by splitting a strippable and splittable composite short fiber comprising at least two resin components which are incompatible with each other;
(ii) the fine fibers should have a monofilament size of 0.01 to 0.5 denier;
(iii) the fine fibers should form a fine non-woven fabric structure that they are entangled with one another at random;
(iv) the sheet should have a weight ratio of the non-woven fabric to the elastic polymer of 97:3 to 50:50;
(v) the sheet should have an apparent density of 0.20 to 0.60 g/cm3;
(vi) the sheet should have an average area of spaces between fibers in the cross section of the non-woven fabric impregnated with the elastic polymer measured by the image analysis of an electron scanning microscope of 70 to 120 xcexcm2; and
(vii) the sheet should have such a uniform structure that the standard deviation of the area of a space between fibers in the cross section of the non-woven fabric impregnated with the elastic polymer measured by the image analysis of the electron scanning microscope is 50 to 250 xcexcm2.
It has been found from studies conducted by the present inventors that the sheet is produced by the following sheet production processes (I) and (II) industrially advantageously.
Sheet Production Process (I):
A sheet production process comprising the following steps:
(1) forming card webs from a strippable and splittable composite short fiber comprising at least two resin components which are incompatible with each other and at least one component of which is heat shrinkable and layering the card webs (layering step);
(2) entangling and stripping/splitting the obtained layered web to split the composite short fiber into fine fibers having a monofilament size of 0.01 to 0.5 denier and entangle the fine fibers with one another so as to produce an unshrunk non-woven fabric (entangling and splitting step);
(3) heating the obtained unshrunk non-woven fabric to shrink the heat shrinkable fine fibers contained in the fine fibers to reduce the area of the non-woven fabric by 10 to 50% (shrinking step); and
(4) impregnating the obtained non-woven fabric with an elastic polymer (impregnation step).
Sheet Production Process (II):
A sheet production process comprising the following steps:
(1) forming card webs from a strippable and splittable composite short fiber comprising at least two resin components which are incompatible with each other and at least one component of which is heat shrinkable and layering the card webs (layering step);
(2) entangling and stripping/splitting the obtained layered web to split the composite short fiber into fine fibers having a monofilament size of 0.01 to 0.5 denier and entangle the fine fibers with one another so as to produce an unshrunk non-woven fabric (entangling and splitting step);
(3) impregnating the obtained non-woven fabric with an elastic polymer (impregnation step); and
(4) heating the obtained unshrunk sheet to shrink the heat shrinkable fine fibers contained in the fine fibers to reduce the area of the non-woven fabric by 10 to 50% (shrinking step).
The above sheet production processes (I) and (II) differ from each other in that the unshrunk non-woven fabric is heated and then impregnated with an elastic polymer in the former process whereas the unshrunk non-woven fabric is impregnated with an elastic polymer and then shrunk by heating in the latter process. The sheet of the present invention is obtained by any one of the processes but the former process is preferred because a structure having finer and more uniform spaces between fibers is obtained.
The sheet and its production process of the present invention will be described in more detail hereinunder.
The elastic polymer to be impregnated into the non-woven fabric (or unshrunk non-woven fabric) of the present invention may be an elastic polymer which is generally used for artificial leather. That is, illustrative examples of the elastic polymer include synthetic resins such as polyvinyl chloride, polyamides, polyesters, polyester-ether copolymers, polyacrylic acid ester copolymers, polyurethanes, neoprene, styrene-butadiene copolymers, silicone resins, polyamino acids and polyamino acid polyurethane copolymers, natural polymer resins, and mixtures thereof. A pigment, dye, crosslinking agent, filler, plasticizer and various stabilizers may be further added as required. A polyurethane or a blend of a polyurethane and other resin is preferably used because a soft feel is obtained.
The above elastic polymer is impregnated into the non-woven fabric of the present invention as an organic solvent solution, dispersion, aqueous solution or water dispersion. To coagulate the elastic polymer, conventionally known processes may be employed. The conventionally known processes include, for example, a drying process, preferably a heat sensitive coagulation process, more preferably a multiple-aperture coagulation process in which a W/O type emulsion is dried. There is also a wet process in which an elastic polymer is multiple-aperture coagulated from an organic solvent having compatibility with water in a coagulation bath essentially composed of water. Any one of the conventionally known processes may be employed.
The amount of the elastic polymer to be impregnated can be easily controlled by adjusting the concentration of the elastic polymer in an impregnation solution and the wet pick-up of the impregnation solution at the time of impregnation. In the present invention, the weight ratio of the non-woven fabric to the elastic polymer is 97:3 to 50:50, preferably 95:5 to 60:40. When the proportion of the elastic polymer is smaller than 3 wt %, a soft sheet is easily obtained but a sheet which is tight and has adhesion strength when an elastic polymer film is formed on the surface to produce a grain type artificial leather is hardly obtained. When the proportion is larger than 50 wt %, the obtained sheet has the strong properties of the elastic polymer and high rubber elasticity and hence, is not suitable as a sheet for artificial leather.
A very tight sheet can be obtained from the non-woven fabric of the present invention even when the amount of the elastic polymer impregnated is small because fibers are finely and uniformly entangled with one another. The impregnated non-woven fabric sheet of the present invention has an apparent density of 0.20 to 0.60 g/cm3, preferably 0.25 to 0.55 g/cm3. The apparent density of the impregnated non-woven fabric (sheet) is determined by the apparent density of a non-woven fabric used and the amount of an elastic polymer impregnated. When the apparent density is lower than 0.20 g/cm3, the uniform structure of the present invention is hardly obtained, high tightness is not felt, and required strength is hardly obtained. Therefore, the obtained sheet is not suitable as a substrate for artificial leather. When the apparent density is higher than 0.60 g/cm3, high tightness is easily obtained but softness and drapeability are hardly obtained.
The impregnated non-woven fabric (sheet) of the present invention is fine and uniform. These characteristic properties are measured by the image analysis of an electron scanning microscope like a non-woven fabric. That is, the average area of spaces formed by the fibers and the elastic polymer in the cross section in a direction perpendicular to the surface of the impregnated non-woven fabric (sheet) of the present invention is 70 to 120 xcexcm2, preferably 80 to 110 xcexcm2 and the standard deviation at this point is 50 to 250 xcexcm2, preferably 70 to 200 xcexcm2. When the average area of the spaces is larger than 120 xcexcm2, the sheet does not become sufficiently fine and the obtained artificial leather is easily wrinkled by bending. When the average area of the spaces is smaller than 70 xcexcm2, the sheet becomes too fine and very tight but hardly obtains softness and drapeability. The value of standard deviation indicating uniformity is preferably small. When the standard deviation is larger than 250 xcexcm2, this means that large spaces are scattered even when the value of average area falls within the range of the present invention. In this case, the obtained artificial leather is easily wrinkled by bending.
The sheet of the present invention has a thickness of 0.3 to 3.0 mm, preferably 0.5 to 2.0 mm.
As for processes for producing the above sheet, the production process (I) which comprises producing a shrunk non-woven fabric by heating an unshrunk non-woven fabric and impregnating the fabric with an elastic polymer has been mainly described. The production process (II) can be also employed without changing the basic conditions and means of each step. That is, in the production process (II), the unshrunk non-woven fabric obtained in the same manner as in the production process (I) is impregnated with an elastic polymer and then the obtained impregnated unshrunk sheet is shrunk by heating. In this production process (II), the heat shrinkage of the heat shrinkable fine fiber is carried out by the procedure and conditions of the production process (I) (procedure and conditions explained in the non-woven fabric production process). However, since heating is carried out after the impregnation of the elastic polymer in the production process (II), in consideration of the fact that the elastic polymer is already impregnated into the spaces between fibers, the heat shrinkage of the heat shrinkable fine fiber and the formation of the fine and uniform spaces between fibers occur as a matter of course but occur less markedly than in the production process (I). Therefore, the results of the analysis of an electron scanning microscope image of the sheet obtained by the production process (II) show that the average area of the spaces is shifted to a slightly higher level in the range of 70 to 120 xcexcm2 and the standard deviation is shifted to a slightly higher level in the range of 50 to 250 xcexcm2.
The sheet produced by any one of the above production processes of the present invention is advantageously used as a substrate for artificial leather. When the surface of the sheet is directly raised, a suede type or nubuck type artificial leather can be obtained. At this point, the value of this sheet can be further enhanced by dying. Further, a grain type artificial leather can be obtained by forming an elastic polymer film on the surface. Conventional grain type artificial leathers are easily wrinkled by bending because their impregnated non-woven fabric as a substrate does not have a fine and uniform structure. Therefore, bending wrinkles are formed by rubbing in advance or an elastic polymer layer formed on the surface is made thicker than required to make up for the above defect. In contrast to this, artificial leather comprising the sheet of the present invention as a substrate is hardly wrinkled by bending, tight and soft, and has drapeability irrespective of the thickness of an elastic polymer film formed on the surface as a grain layer.
To form the elastic polymer film on the surface as a grain layer, conventionally known methods are employed. Typical methods include a lamination method in which an elastic polymer film is formed on release paper and bonded to the surface of an impregnated non-woven fabric with an adhesive, one in which a grain layer is formed by coating a W/O type emulsion of an elastic polymer on the surface of an impregnated non-woven fabric, drying the coating film to form a porous layer, and embossing or gravure coating, one in which a film is formed on the surface of the porous layer by lamination, one in which a grain layer is formed by embossing or gravure coating a porous layer formed by a wet process in which a water-compatible organic solvent solution of an elastic polymer is coated on the surface of an impregnated non-woven fabric and multiple-aperture coagulated in a coagulation bath essentially composed of water, and one in which a film is formed on the surface of the porous layer by lamination.
The artificial leather of the present invention obtained as described above can be favorably used in a wide variety of application fields such as upper materials and auxiliary materials for sports shoes, balls such as soccer, basket and volley balls, bags such as trunks, handbags and attache cases, sheets for couches, chair linings and cars, gloves such as golf gloves, baseball gloves and ski gloves, and garments after its softness, surface pattern, color and luster are adjusted. Since the artificial leather of the present invention has softness and excellent physical strength and is light in weight and hardly wrinkled by bending, it has great value as an upper material for shoes, especially as an upper material for sports shoes. Further, it can be advantageously used for balls, furniture sheets, car sheets, garments, gloves and pouches such as trunks and bags.